DPCM coding of video signals has been improved by the use of variable length coders which assign code words of different lengths to each quantizer prediction error output, depending upon its expected frequency of occurrence. Because variable length coding generates data at an uneven rate, buffers must be provided at both transmitter and receiver so that information can be transmitted at a constant bit rate on the interconnecting communication channel. The buffer smooths the coder output data rate by reading data at a variable rate while writing data at a regular rate dictated by channel capacity. In order to avoid overflow, buffer fullness is typically sensed and used to adaptively control the DPCM coder, for example, by changing the quantizer characteristics (see U.S. Pat. No. 4,077,053 issued to T. Ishiguro on Feb. 28, 1978) or by interposing a variable characteristics filter in the DPCM coder input (see U.S. Pat. No. 4,047,221 issued to H. Yasuda et al on Sept. 6, 1977). Buffer overflow can also be controlled by resorting to spatial and/or temporal subsampling so that the volume of data that must be processed is reduced in picture areas which generate large prediction errors.
When spatial subsampling is used in DPCM coding, information describing the nontransmitted picture elements must nevertheless be computed at the receiver, for display purposes. Nontransmitted pels must also be reconstructed at the transmitter, since both the transmitted and nontransmitted pels are normally used in predicting the values of subsequent pels. Reconstructed values for nontransmitted pels have typically been derived by interpolation, using the reconstructed values of neighboring transmitted picture elements. This is adequate in many instances, as where there is motion or a high degree of spatial detail in the picture. However, since interpolation is an averaging process, it produces noticeable blurring in flat or stationary picture areas, which is often highly objectionable.